New approaches to the treatment of asthma

Overview

Prejudices from the ancient past resonate in how asthma is treated in the modern day. Inhalation therapies date as far back as the earliest civilisations. Thanks to improvements during the 20th century, the symptoms of asthma can now often be managed. A clear understanding of the disease, however, has remained elusive through the ages.

Since the 1990s, immune system cells and signals have been emerging as an underlying cause for shortness of breath, airway hyper-responsiveness and constriction of the airway smooth muscle. Following the thread of immunology, researchers implicated a specific immune response meant for eliminating invading pathogens. This response, when occurring in the lung, can lead to an imbalance in the immune system and to chronic inflammation.

Scientists at AstraZeneca are working to develop a therapy designed to restore a healthy balance to the immune system of asthma patients. The immune system uses a group of pattern recognising receptors to identify pathogens and activate a response to eliminate them. The discovery of these receptors, known as the toll-like receptors or TLRs, provided a key target for attempting to correct the imbalanced immune response associated with asthma.

Ten different types of receptors have been identified. Years of extensive internal and external research efforts helped to narrow the target selection to TLR9. Now, AstraZeneca is working with Dynavax, a biotech company that specialises in TLR biology. Together these companies are developing an agonist that interacts with TLR9 to attenuate the pathogenic immune response that causes asthma.

The hygiene hypothesis

The worldwide incidence of asthma is rapidly increasing. The hygiene hypothesis – less exposure to microbes and infections as a child – may provide one explanation for why the world may be more prone to the disease.

Over the last 40 years, the world has witnessed a sharp increase in the global prevalence of asthma. An estimated 300 million people worldwide suffer from the disease. Expectations are that the number will continue to increase by 50% each decade1. The cause for the rapidly rising incidence of asthma is far from being completely understood. The hygiene hypothesis, however, may provide one explanation. The theory points to a reduced exposure to childhood infections as an explanation for the increased prevalence of allergic diseases in industrialised countries2. A cleaner upbringing associated with modern times may prompt an imbalance in the immune system for some individuals3.

According to the World Health Organisation (WHO), about 70% of people with asthma also have allergies. Several types of allergic asthma exist. Each is defined by the factor prompting the condition. In addition to allergens, symptoms of asthma may be triggered by: irritants in the air, a respiratory infection, and even exercise and some medications. Asthma symptoms can often be managed using medication. Most asthma medications, such as inhaled glucocorticoids, temporarily alleviate symptoms of asthma.

02

An ancient malady

Asthma is documented in the treatises of ancient civilisations. Not until the 20th century did scientists come closer to understanding what causes the disease.

Asthma is a persistent malady that has plagued humanity since early civilization. Despite documented awareness of asthma dating back to ancient civilizations (Egyptian, Greek and Chinese), a cure for the disease has not yet been found.

Inhalation therapies have been used throughout history. Two ancient remedies, belladonna herbs and the Ma Huang plant, have even provided extracts that are still used as drugs (belladonna alkaloids and ephedrine). Neither is a 1st line treatment. Through the centuries, the immune system remained an elusive culprit and the development of medicines advanced with a limited view of what causes sufferers to gasp for breath. Therapies that manage the shortness of breath have marked the greatest improvement in the care for asthma.

In the 1980s investigators established strong links between the inflammation of the airways and asthma4 . Advances in the understanding of asthma that followed implicated specific cells and chemical signals from the immune system. These cells and signals play a central role in causing the different types of asthma. It is this avenue that AstraZeneca is following in pursuit of potentially longer-lasting treatments.

Toll, asthma’s death knell?

The discovery of toll-like receptors TLRs pave the way for finding a potential cure for asthma.

The immune response to pathogens is orchestrated by the complex interactions and activities of the large number of diverse cell types, chemical signals and receptors. Pattern recognising, toll-like receptors (TLRs) detect telltale signs, such as foreign DNA or RNA, from an invading pathogen. After detecting a pattern, TLRs prompt the immune system to initiate an innate and adaptive response that clears the infection.

Innate immunity is the immune system’s rapid response arm: the cells and proteins are ever-present and ready to mobilise and fight microbes within hours of infection. Phagocytes and natural killer cells are examples of cells found in the innate response. In contrast, an adaptive response takes a few days to rev up. This is due, in part, to the time it takes for naïve B-cells and T-cells to proliferate, mature and adapt to the infectious agent. T-cells have been linked to the pathology of asthma.

Some asthma patients show elevated levels of type 2 T-helper cells (Th2)5. Asthma may result from an inhaled allergen inducing Th2 cells to drive inflammation. The response is really meant to keep extracellular parasites in check. When unleashed continuously in the lungs, it can cause airway hyper-responsiveness, excessive mucus secretion and inflammation of the airway3. All are symptoms of asthma.

A spate of landmark discoveries6-11 made it clear that TLRs can direct the action taken by the immune system. When the news of a new subclass of immune receptors emerged, scientists at the former AstraZeneca Charnwood research site in the UK began to explore TLRs for their therapeutic potential. The early research that took place in Charnwood was an effort to discover novel molecules that would activate toll-like receptors in order to study their role in immunology. These molecules could also serve as a starting point for drug discovery.

Ten distinctive types of human TLRs had been characterised6. “We worked on about 6 of them,” elaborates Dr Delaney. “We had a number of toll programmes and whittled it down based on the data.”

“We built up a lot of experience in the toll receptor area. It became clear we would have to go into different modalities based on the difficulty of obtaining good ligands,” Dr Delaney adds. This meant looking for partnerships to provide the expertise. “As the story unfolded with the two clinical development programmes that continued, we were looking into quite new areas in the chemical space,” Dr Delaney says.

AstraZeneca partnered with two companies. A research collaboration with Sumitomo Dainippon Pharma Co. Ltd. Japan led to the joint invention of a small molecule (AZD8848) agonist for TLR7. Dynavax, a company using DNA analogues to reach toll receptors, provided the uncharted territory of an oligonucleotide agonist for TLR9 (AZD1419).

The ability to modulate TLR7 or TLR9 provided a new way to address the underlying pathology of asthma. Remember the Th2 cells that caused the lungs to clog up with mucus and contract – both receptors set in motion an immune cascade that downregulates Th2. Competition between type 1 T-helper cells (Th1) and Th2 cells means, when stimulated, each of these T-cells can stifle a response by the other. When the Th1 inflammatory response is underway, chemical signals or cytokines are released to modify a Th2 inflammatory response, to make it more Th1-like12. TLR7 and 9 promote the Th1 response. Modulating these receptors provides a path to restoring balance and steering the immune response away from the Th2 dominance seen in many asthma patients. However only one of the clinical trials programmes is still ongoing.

Not so lucky 7

Despite best efforts in the TLR7 area our development candidate proved unsuccessful.

“In clinical development, we learned a lot about the human biology,” explains Dr Delaney. Observations from the clinical trials conducted for AZD8848 showed the drug could reduce responsiveness to allergens in allergic rhinitis13 even after repeated allergic challenges. These initial human trials used intranasal delivery.

“By dosing in the nose, we were using the united airways model,” says Dr Delaney who is referring to the idea that efficacy observed in the nose could be linked to efficacy in the lungs. "We learned, however, that there were differences in the immunology. Delivery to the nose will not give a sufficient effect in the lungs.”

As AZD8848 became an inhalation therapy to be delivered directly into the lungs, the balance between the tolerability of the therapy and the clinical benefit became clearer. “These are powerful mechanisms and we wish to focus their effect on the lung,” Dr Delaney emphasises. TLR7 recognises the single stranded RNA associated with viral infection, such as influenza. As AZD8848 is designed to mimic a virus triggering the immune system, it can be expected to induce flu-like symptoms. If accompanied by long-term relief from asthma symptoms, an asthma sufferer might find transient flu-like symptoms tolerable. “When there is a product, we have to have a good proposition for someone, in feeling like they have the flu for a few hours, relative to months of relief from their asthma,” explains Dr Delaney. Trials to determine the safety and tolerability of inhaled doses of AZD8848 unfortunately did not demonstrate a clear window between stimulation of TLR7 in the lung and flu-like side effects, and the compound’s progress was halted.

“Our toll-like receptor 7 agonist did not have the correct balance relative to side effects”, explains Dr Delaney. This left our TLR9 compound AZD1419 as our only clinical candidate.

05

Following the TLR9 path

A therapeutic candidate, AZD1419, designed to interact with TLR9, is currently in clinical trial development.

The pathogenic trigger for TLR9 differs from TLR7. The TLR9 receptor detects a cytosine-phosphate-guanine sequence common to bacterial DNA. Once the sequence is detected, TLR9 kicks off an attack involving at least three different types of lymphocytes. B-cells proliferate, natural killer cells activate, and the Th1 inflammatory helper T-cell response is stimulated.

“It was important for both TLRs to ensure that the optimised drug could be inhaled directly into the lung with little or no effect on TLRs in other parts of the body. Thus we had two very different types of molecule targeting two different TLRs. From early studies in healthy volunteers, we found that the better combination was the TLR9-oligonucleotide and thus AstraZeneca's efforts became focused on AZD1419 and the collaboration with Dynavax,” outlines David Keeling, Project Director, IMED Biotech Unit (Respiratory, Inflammation & Autoimmunity).

TLR9 emerged a suitable target because, like TLR7, it should tip the immune system away from the Th2 dominance in asthma. “The science is not all inside AstraZeneca,” says Dr Keeling, referring to the research and clinical development partnership with Dynavax. Robert Coffman, who is the Chief Science Officer at Dynavax, helped to make some of the seminal discoveries that led to a better understanding of the immune system and its relationship to asthma.

In 1986, Dr Coffman and Dr Tim Mosmann14 (his colleague at the time) published the findings that first defined the two principal subtypes of helper T-cells (Th1 and Th2). Dr Coffman also went on to disclose the basic mechanisms of T-cell regulation in asthma.

“For 15 years, many of us plugged away with limited success,” Dr Coffman notes. “I became aware of the technology being developed at Dynavax. Early experiments conducted by several groups demonstrated that this technology showed some real promise.”

Dynavax specialises in targeting toll-like receptors with oligonucleotides for the treatment of disease. “In the Dynavax collaboration, we were working with a novel group of compounds, modified oligonucleotides, and identified one that was suitable for progression to the clinic,” says Dr Keeling, referring to AZD1419.

Dynavax’s oligonucleotides are structurally similar to naturally occurring DNA. AZD1419 is a synthetic analogue that mimics the bacterial DNA pattern detected by TLR9. By activating the innate immune system in a way that mimics bacteria, an agonist interacting with the TLR9 receptor, such as AZD1419, can be used to reverse, redirect or suppress the overzealous Th2 response associated with asthma15.

“It was a true collaboration with their researchers to develop a sequence motif that was optimised to inhibit allergic responses,” emphasises Dr Coffman. When he joined Dynavax in 2000, the pursuit of a clinical program was undertaken to target TLR9. “We had sequences that seemed to be suitable. Those trials gave us reason for hope. But we did not have an optimal toll receptor targeting sequence.” The dynamics of being a small biotech company meant the clinical program stalled. “We made relatively little progress. The breakthrough was partnering with AstraZeneca.”

AZD1419 is inhaled directly to the lung in the hope that direct delivery will maximise the desirable effect whilst minimising the potential for side effects, which might result from stimulating TLR9 beyond the lung. “AstraZeneca has extensive knowledge of what it takes to deliver a medicine directly to the lung and by combining this with Dynavax’s expertise in immuno-stimulatory oligonucleotides we have a winning team,” says Dr Keeling.

Developing therapies to target toll-like receptors constitutes a new class of treatment. “The real excitement is that in pre-clinical models we see the inflammation in the lung disappear. And over time, we even see it knock the immune system back into balance," describes Dr Keeling.

Meet the team

Despite promising early clinical results in healthy volunteers, the desired effect has yet to be demonstrated in asthma patients. Phase 2 clinical trials are in the planning stages. How does Dr Keeling feel about the odds of success?

“In pre-clinical models you start to see that the lung is not inflamed. But biology can be frustratingly complex, we don’t yet know if it will work in the same way when we conduct a study in patients,” he emphasises.

The science surrounding toll-like receptors constitutes new ground and the team at AstraZeneca is dedicated to pursuing any possibility that could benefit patients with asthma. Dr Keeling asserts, "The company knows this is really exciting science and it is ground-breaking. Wanting to change the disease, rebalance the immune system – that is what we are going for.”

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